1. Field of the Invention
The present invention relates to a motor speed detection apparatus that detects rotational speed of a motor by using output from a frequency generation coil.
2. Description of the Related Art
A method of performing feedback control of rotational speed (rotational frequency) of a brushless motor has been proposed which measures the rotational speed using a frequency generation coil (hereinafter referred to as “an FG coil”). Specifically, this method provides the FG coil and a magnet that are relatively moved with rotation of the motor, and samples an alternating-current signal output from the FG coil to detect (calculate) the rotational speed of the motor.
Such a feedback control method can feed back change in rotational speed of the motor more accurately as a sampling frequency for the output of the FG coil (hereinafter referred to as “FG coil output”) is set to be higher, which enables good feedback control of the motor.
Japanese Patent Laid-Open No. 08-009670 discloses, as a method of raising the sampling frequency for the FG coil output, a method multiplying (doubling) the FG coil output. This method detects zero crossings of the FG coil output and causes a one-shot mono/multi-vibrator, at each detection of the zero crossing, to produce two pulse outputs for each cycle of the FG coil, thereby providing a raised sampling frequency doubled with respect to an original frequency of the FG coil output. Moreover, Japanese Patent Laid-Open No. 08-009670 discloses, as a method of further raising the sampling frequency, a method differentiating the original FG coil output so as to shift its phase to produce a new FG coil output and doubling the original and new FG coil outputs to combine them, thereby a sampling frequency four times as high as the original frequency of the FG coil output.
However, even the method disclosed in Japanese Patent Laid-Open No. 08-009670 cannot provide a sampling frequency higher than four times of the original frequency of the FG coil output. Providing such a higher sampling frequency requires raising of the original frequency of the FG coil output itself, but it is difficult to raise the original frequency of the FG coil output because of physical restriction in the FG coil and magnet.